Calibration and Annual Changes of the Coefficients of the Angstrom-Prescott (A-P) Equation (a and b) in Different Time Scales (Case Study: Tehran North Station (Aghdasieh))

Angstrom-Prescott equation is one of the most commonly used methods for the estimation of global solar radiation (Rs) based on sunshine hours. The critical step in the application of this method's is the calibration of its coefficients for each region. Although the coefficients of the equation have been calibrated in different parts of the World, the effect of different time scales has not been considered clearly. In this article, variations of the coefficients of A-P equation in different time scales and their effects on the estimation of global solar radiation are studied. For this purpose data for the 15-year period of 1992-2006 from the Tehran-North (Aghdasieh) station in daily and monthly time scales are used. The values of the coefficients derived from the daily and monthly data were clearly different, and the equation based on monthly data has higher coefficient of determination (R2=0.92) compared to that based on the daily data (R2=0.48). We also found that the daily total solar radiation incident at the surface can be estimated well by using the A-P equation derived based on monthly data.